Include the $Id$ CVS vendor tag in each file. As each file is edited, add this tag if it's not yet present (or replace existing forms such as "Last Modified:", etc.).
Note: We have a custom $Horde tag in Horde cvs to track our versions seperately; we could do the same and make a $PEAR tag, that would remain even if PEAR files were put into another source control system, etc...]